The present invention relates to intrusion detectors and in particular relates to a new arrangement and method for processing the signals received from passive infrared detectors.
Passive infrared intrusion detectors are often used in combination with a microwave detection arrangement to provide a detector having a dual technology which is less prone to false alarms and is generally considered more reliable. Various types of detectors are often combined to provide improved reliability and increased sophistication. The present invention will be discussed with respect to single infrared intrusion detectors, however, it would be appreciated by anyone skilled in the art that this arrangement can be used in any system using a passive infrared detector or other detector or system having a similar type signal.
The signal from a passive infrared detector with respect to the disturbances which occur in the area being monitored is an alternating signal sometimes considered predominantly sinusoidal whose magnitude typically varies between 0 and 3.6 volts peak to peak (5 volts supply) and whose frequency varies from 0.1 to 10 hertz.
Some approaches for analyzing this signal include the use of two comparators, one for evaluating positive portion of the signal and the other for evaluating the negative portion. Pulses are produced when the signal exceeds the threshold of the respective comparator and are of a duration corresponding to the time that the signal remains above the minimum threshold. Thus positive pulses of variable duration have been derived by use of two comparators for evaluating positive and negative portions of the signal from the infrared detector. It is also possible to rectify the signal and merely use a single comparator for evaluation of the signal. The problems with the comparator approach is that it is difficult to determine what the best minimum threshold is. A number of factors can affect the signal from the detector and not all of these disturbances indicate that a burglar or intruder is present. RF transient signals produced in walkie-talkies, etc. can produce a very strong short duration signal. Heaters coming on within the monitored area can produce a detectable signal as well as small animals such as a cat, etc., crossing through the zone. Therefore, a problem arises in trying to distinguish between the presence of a human intruder and a disturbance in the signal which is not produced by such an intruder.
A different approach has been to integrate the output signal as this integration is in effect the measurement of the energy of the signal and it is believed this measurement is more indicative of whether an intruder is present. Unfortunately other factors enter into the consideration such as the ability of the system to detect the desired intruder at a long distance from the detector which typically produces a fairly low frequency signal. Other problems also occur due to the widely varying ambient temperature conditions that can occur in the monitored area.
Many systems have used a single comparator to produce a pulse which is counted, and if sufficient pulses are produced within a certain time period an alarm condition is produced. Counting arrangements can produce false alarms as common environmental disturbances such as blasts of hot air from the heating vents will produce the same unit of information as the sensing of a valid target. In order to reduce the occurrence of false alarms it is possible to increase the comparator trip threshold and/or increase the number of pulses counted before an alarm is generated. Both of these techniques will indeed improve the false alarm immunity however this will be accomplished at the expense of the detection range of the unit. If the number of pulses counted before an alarm condition is produced is increased far detection range will be decreased since far targets will produce few pulses (due to low amplitude and frequency). If the thresholds are increased, far response will again be reduced since the far signals are of lower amplitude. It is for these reasons that maximum pulse setting allowed is typically 3.
In one prior art arrangement the output signal from the detector is fed into an absolute value circuit and subsequently to a voltage controlled pulse generator subsection. When the signal reaches a minimum amplitude the voltage controlled pulse generator begins to produce constant width pulse at a repetition rate proportional to the amplitude of the signal typically in the hundreds of hertz. These pulses are counted or integrated and stored by the means of a capacitor. When the stored energy reaches a preset level an alarm signal is generated. This system suffers the same basic draw backs as a window comparator system in that slowly changing low amplitude transients which barely cross over the threshold generate full amplitude pulses which are integrated towards a possible alarm generation.
Since the slow transients are allowed to produce the same unit of information as valid distance targets the low frequency response of the amplifier has been set to de-emphasize low frequency response to reduce the probability of false alarms. Unfortunately since distant valid targets produce low frequency signals the overall pattern coverage is decreased as a result.
According to a different arrangement the sinusoidal signal is fed into an absolute value circuit and when this signal exceeds a minimum threshold its amplitude is used to vary the charge current of a capacitor which is used as a energy storage device. The charging current equation is EQU I.sub.charge =(V.sub.signal -V.sub.minimum threshold)/R.sub.charge
When a certain amount of energy over time (in volt seconds) has been accumulated in the capacitor the unit will signal an alarm. This technique is an improvement over previous methods in that the effects of low amplitude transients which barely cross over the minimum threshold are reduced. This is accomplished as their energy over time is low and thus their contribution to the accumulated total energy is low. This technique does require the gain of the amplifier to be excessively high to quickly generate an alarm condition by far-off targets moving at low speed. This presents a problem for RF induced transients which are greatly amplified as a result of this excessive gain requirement.
The present invention seeks to overcome the problems associated with the prior art techniques and provide a system having improved information processing allowing more accurate evaluation of the signal. The invention in the simplest form is relatively inexpensive but the system is also capable of a high degree of sophistication and evaluation of the signal for more demanding applications.